A radiation image recording and reproducing method utilizing a stimulable phosphor described, for instance, in U.S. Pat. No. 4,239,968, is now practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a transparent film of polymer material is generally placed on the free surface (i.e., surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor dispersed therein. The stimulable phosphor emits stimulated emission when excited with a stimulating ray after having been exposed to a radiation such as X-ray. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the from of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore. It is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.).
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission given by the stimulable phosphor contained therein, and the total emission amount varies depending upon not only the emission luminance of the phosphor but also the content (i.e., amount) of the phosphor in the phosphor layer. The large content of the phosphor also results in increase of absorption of a radiation such as X-rays, so that the panel shows an increased high sensitivity and provides an image of improved quality, especially an image of improved graininess. On the other hand, assuming that the content of the phosphor in the phosphor layer is kept at the same level, if the phosphor layer is densely packed with the phosphor, a panel using such phosphor layer provides an image of high sharpness, because such phosphor layer can be made thinner to reduce spread of stimulating rays caused by scattering in the phosphor layer.
U.S. Pat. No. 4,910,407 discloses a radiation image storage panel having a compressed phosphor layer provided on the support. Since the compressed phosphor layer is packed with the phosphor more densely than conventional phosphor layers, the panel disclosed in the publication gives an image of improved sharpness. However, in contrast, the obtained image is often rendered poor in view of graininess because the compression treatment destroys a part of the phosphor in the layer. In order to solve this problem, Japanese Patent Provisional Publication No. H2-278197 proposes a radiation image storage panel having a compressed phosphor layer containing a particular binder. In more detail, a thermoplastic elastomer having softening point or melting point of 30 to 150.degree. C. is used as a binder of the phosphor layer, and the compression treatment is carried out at the temperature above the softening point or melting point. Since this compression treatment makes the phosphor densely packed in the phosphor layer without destroying, the panel gives an image of both high sharpness and high graininess. Further, Japanese Patent Provisional Publication No. H7-287098 proposes a radiation image storage panel having two phosphor layers comprising different binders of thermoplastic resin (for example, thermoplastic elastomers having different softening points).
In the radiation image recording and reproducing method, the radiation image storage panel is repeatedly used in the cyclic procedure comprising the steps of exposing to a radiation (for recording of a radiation image), irradiating with stimulating rays (for reading of the recorded image) and exposing to an erasing light (for erasing the remaining image). In an apparatus for this method, the panel is repeatedly transferred from one step to another step by means of conveying means such as belt and rolls. Such repeated conveying, however, is liable to cause some cracks in the phosphor layer especially when the panel has the above-described phosphor layer compressed under heating. Since the cracks are apt to scatter the radiation and/or stimulating rays, the panel having a cracked phosphor layer gives an image of poor quality. In order to solve this problem, U.S. Pat. No. 5,641,968 proposes a further improved radiation image storage panel. In the proposed panel, the binder of the phosphor layer comprises a thermoplastic elastomer (e.g., polyurethane elastomer) having an elastic modules of not more than 0.3 kgf/mm.sup.2, as well as a softening point or melting point of 30 to 150.degree. C.
As described above, thermoplastic polyurethane elastomer is known to have excellent properties as a material for the binder resin of the phosphor layer of the radiation image storage panel, especially for that of the phosphor layer compressed (after having been formed) under heating so as to be densely packed with the phosphor.